Inspection of vehicles and containers by imaging backscattered radiation has, in the past, used mechanical means to create beams of x-rays that scan the targets. Various schemes for electromagnetically scanning x-ray beams are known as well, as taught, for example, in U.S. Pat. No. 6,249,567 (to Rothschild et al., 2001), which teaches the use of an electromagnetically scanned x-ray beam to scan the undercarriage of a vehicle.
FIG. 1 shows a prior art X-ray backscatter system 100 where a source of x-rays 102 and a set of x-ray backscatter detectors 104 are enclosed within an inspection vehicle 106 that is moving relative to a target 108 (otherwise referred to herein as an “inspection target”, an “inspected target” “target vehicle,” or “inspected vehicle” (or “car” or “truck,” as the case may be). A typical direction of relative motion is indicated by arrow 116, where either the inspection vehicle 106, or the target vehicle 108, or both, may be in motion relative to the surroundings. The basic elements of the backscatter system shown in FIG. 1 include an x-ray beam former 20 (shown in FIG. 2), one or more backscatter x-ray detectors 104, a signal processor 110, and a user interface 112. Source 102 includes beam former 20 (otherwise referred to herein as a “mechanical scanner”), which forms x-rays into a pencil beam 201 (shown in FIG. 2) that is swept in a scanning pattern 114 that is typically in a vertical plane. A prior art beam former is shown in FIG. 2 and designated there generally by numeral 20, and described in detail in U.S. Pat. No. 9,014,339 (hereinafter “Grodzins '339”), which is incorporated herein by reference. Beam former 20 consists of an x-ray tube 203, in which a fixed beam of electrons 205, emitted by a filament 207 at a negative high voltage, is focused to a spot on a reflection anode 209. The x-rays, constrained into a fan beam by a collimator 211, impinge on a rotating hoop 213 that has N equally spaced apertures 215 (N=4 in FIG. 2) that produce pencil beams 201 of x-rays that sweep across the target (vehicle 108 in FIG. 1) N times in each revolution of the hoop 213. The x-rays that are backscattered by Compton interactions in the target vehicle 108 are detected by large-area backscatter detectors whose signals are processed into images as the car (i.e., target vehicle 108) moves through the scanning pencil beam 201.
The specifications of the scanning pencil beam 201—intensity, sweep speed, sweep angle, resolution, etc.—are determined by the parameters of the x-ray tube 203 and mechanical scanner 20. In cases of backscatter systems deployed on inspection vans 106 and used to inspect vehicles 108, as shown in FIG. 1, it is standard practice to design the mechanical scanner (which term is used synonymously herein with the terms “beam former” and “chopper”) 20 to give optimal image quality for a specific height of vehicle that moves at a specific drive-by speed and specific distance from the inspection van. Vehicles of other heights or different distances or different speeds will be inspected under less than optimum conditions.
FIG. 3 illustrates a prior art example of less-than-optimal matching of a beam-scanning system to a particular vehicle under inspection. In the prior art scenario depicted in FIG. 3, a stationary inspection-van (not shown), inspects a car (inspected vehicle 108) moving at 5 kph, at a distance of 5 feet from the chopper 20. The prior art chopper hoop 213 of FIG. 2, having a wheel diameter of 24 inches, with four apertures 215 of 1.5 mm diameter each, spins at 40 revolutions per second, creating successive 90° sweeps, each taking 6.25 msec. The 1.5 mm aperture at a distance of 12 inches from the x-ray source produces a 9 mm wide pixel at 5 feet, the minimum distance to inspected vehicle 108. During each sweep of the beam, the inspected vehicle has moved a distance of 8.7 mm, so successive sweeps abut and overlap, such that the car is fully scanned.
FIG. 4A is a beam coverage plot of successive beam sweeps that follow one another as the hoop of FIG. 2 rotates. For heuristic simplicity in making the point of this paragraph, it has been assumed that the pixel width is 9 mm and unchanged during the sweep. In fact, the sweeps may form an hourglass, with pixel widths 40% wider at the top and bottom of the 90° sweeps, for example. The uniform widths of the scanned swaths in each of FIGS. 4A-4C are typical representations of the sweeps on a typical beam former, although that uniformity imposes undesirable limitations for x-ray inspection applications, as will now be discussed.
Using the values that have been discussed above for purposes of demonstration, the pixel width is always greater than the distance of 8.7 mm (rounded to 9 mm in FIG. 4A) that the car moves during an inspection. The car is fully scanned. At a higher speed, 8 kph, for example, the car moves almost 13 mm during each sweep so that the pixel pattern shown in FIG. 4B has gaps. The car is seriously under-sampled. FIG. 4C shows the pattern for a van speed of 2.5 kph. (The vertical offset of the scans as shown is for illustrative purposes only.) In the latter case, the beam width is at least twice the distance the vehicle has moved in a sweep and successive scans fully overlap. The oversampling by a factor of two improves the statistics of the measured intensities but at a cost of doubling the time of an inspection. Under-sampling or oversampling also results when the car, moving at 5 kph, is closer or further from the inspection van.
It is to be noted that the 90° scan beam of FIG. 3 was designed for optimum coverage for a 12-foot truck at a distance of 5 feet. A car at a distance of 5 feet is fully scanned, but more than 50% of the beam has been wasted, scanning air.
Mechanical methods have been suggested to change the scan parameters between successive inspections so as to zoom the full x-ray beam on to the target L, and are described in Grodzins '339. But mechanical means cannot change beam parameters during the course of the inspection itself. Insofar as no means currently exists to change beam parameters during the course of inspection, techniques for doing so, described and claimed herein, constitute a timely invention.
Means for changing the shape of an x-ray beam by electronically varying the shape of an e-beam as it impinges upon a Bremsstrahlung target have long been known, and have been described in such references as U.S. Pat. No. 5,822,395 (to Schardt et al., 1997) where the cross-section of an electron beam is shaped to minimize apparent focal spot distortions for off-center angles, selectable target angles and beam power levels. Various electromagnetic scanning systems have also been taught where the propagation direction of an emergent x-ray beam may be varied electromagnetically. One example is U.S. Pat. No. 6,282,260 (to Grodzins).
Electromagnetic steering of an electron beam in the course of generating an x-ray beam comprises an aspect of the present invention, as discussed below. The prior art has described the discontinuous switching of electron beams among multiple anodes of a multi-anode x-ray tube, an aspect that does not accomplish the objectives of the present invention discussed below.
It had always been indicated, in all known references to the field of x-ray imaging, that obtaining multiple x-ray images of a target during the course of a single scan would require either multiple x-ray sources, splitting an electron beam to a sequence of radiation-producing targets, as suggested in US Published Patent Application US 2011/0206179 (Bendahan), or else employing a fast beam kicker, again to shift an electron beam to multiple individual targets at a high rate, as taught in US Published Application 2013/0136230 (Arodzero).
However, prior to the present invention, no one has ever been able to devise a way to obtain more than a single image of x-ray interactions with a single target during the course of a single pass of the inspection system relative to the inspected object using a solitary source with a solitary Bremsstrahlung target.